U.S. Pat. No. 4,887,530, issued to Jeffrey W. Sainio on Dec. 19, 1989, discloses a control system for adjusting the color-to-color registration of multi-color web-fed printing press systems. In general, the device utilizes a registration mark distinct from the printed image to provide color-to-color registration. An optical scanner scans registration marks each associated with one color of the printed image. The optical scanner provides information to a control system which allows the control system to determine the spatial relationship of the registration marks and control the printing units of the printing press system such that the registration of printed colors is corrected as necessary.
The system of U.S. Pat. No. 4,887,530 is reliable and has proven highly useful for maintaining color-to-color registration of multi-color prints. However, the requirement of a registration mark distinct from the image being printed requires additional paper which is discarded and adds cost to the printing process. Additionally, there are certain types of printed materials which do not provide a convenient area for applying registration marks. Accordingly, it would be advantageous to provide a system which is able to provide color-to-color registration based only upon the image being printed.
U.S. Pat. No. 4,736,680, issued to R. Langdon Wales and H. W. Crowley on Apr. 12, 1988, discloses a register control system for use with a four-color printing press which utilizes a camera to scan a selected area of an image and locate a black dot in the image. In addition to locating the black dot, the system scans the area about the black dot to locate yellow, magenta, and cyan dots. Based upon the known relationship between the black dot, and the yellow, magenta and cyan dots, the system calculates correction values for the X and Y coordinates, which are simply the distance between the theoretical location of the dots and their electronically observed positions. The correction values are utilized by a processor which applies correction values to the printing press to correct for misregistration.
Systems utilizing dots of a printed image to determine misregistration of the colors of a printed image suffer the problem of only being capable of providing registration for half-tone images. They are unable to provide registration for full-tone images. More specifically, in a half-tone image, dots exist since the image does not require printing upon 100% of the area of the web in the area where the image is printed. However, full-tone printing requires application of ink to 100% of the area within which the full-tone image is being printed. Accordingly, a full-tone image does not include dots which can be referenced to provide registration.
Accordingly, it would be advantageous to provide a color-to-color registration system capable of providing registration based upon a portion of a printed image including either half-tone or full-tone printing.